The template is being applied to all emails. the only restriction is the theme must use the WordPress API wp_mail function to send an email.
All AppThemes premium themes do that. though a broken child theme or plugin may not do that and then the template won’t be applied to the specific theme that child theme or plugin may send.
You shouldn’t get into any problem with AppThemes Vantage theme.

What kind of more info are you asking?
the template is configurable from wp_admin as shows. can edit the header and footer texts using html editor. can easy add logo and social links.
You can add any image actually in header/footer using the html editor.

having a question before going for your plugin: I’m using “AppThemes Confirm Email” – is it possible to format these predefined emails aswell? If Yes – it would be great – and wouldn’t hesitate to buy …

I am currently using WP Email Template LITE plugin. I have P3 – Plugin Performance Profiler installed and it reports that it is using too much resources (36%). Do you have P3, and if so can you tell me if your plugin is more efficient?

Also, does this wrap all emails from system including from contact poster?

As for P3. since it’s in percentage it depends on what you have installed. Since my demo site doesn’t have anything installed it could show higher percentage of usage but if I’ll install some heavy plugins then the percentage will be a lot lower. It also depends on the amount of traffic to the website and my demo site doesnt get visitors at all. Also for this plugin it depends on the amount of emails generated.
So you see I can’t really get real result for you as I use non active demo site.
Also someone checking will get real result for his website but it won’t be the same for you.

I don’t have a lite/trial version but you can use any free email styler plugin from wordpress.org.
If it works then this plugin will work as well.
You can try wp-email-template. they have a free lite version.

the plugins work separately. email styler is for nice html template you see in the screenshot and can work with any theme. classipress customs emails is for changing the content of the email and is for classipress only.